1. Field of the Invention
The present invention relates to an improved controller for a wound-rotor induction motor.
2. Description of the Prior Art
The Scherbius apparatus has been used as a variable speed driving controller for a pump or a fan. The Scherbius apparatus uses a wound-rotor induction motor. A secondary winding of the wound-rotor induction motor is connected to a rectifier, and a regenerated secondary power is supplied to an AC power supply via a DC reactor and an inverter. The Scherbius apparatus has an advantage of high controllability because it directly controls a secondary current of the induction motor proportional to motor torque. The secondary power of the induction motor is regenerated to the power supply, so that the speed control is executed efficiently, and even when the converter fails, by switching to the secondary resistance control mode, the operation can be continued, so that it is applied to a public field such as water supply and drainage.
The conventional Scherbius apparatus uses a thyristor for the inverter connected to the power supply, so that it has disadvantages such that the power factor of the inverter is low and the apparatus is affected by voltage fluctuation of the power supply. Furthermore, the secondary current is rectified by a diode bridge to a square wave at a continuity angle of 120°, so that the secondary current waveform includes harmonic ripples, and as a result, a problem arises that motor torque ripples are generated.
In order to avoid such a problem, a controller for a wound-rotor induction motor is proposed in Japanese Patent Publication (Kokai) No. 2003-134871. This controller has a secondary chopper for secondary current adjustment to the output side of the rectifier, connect a series circuit of a diode and a capacitor in parallel with the secondary chopper, operate the secondary chopper as a boosting chopper, thereby improve the power factor of the inverter. And a proposal of multiplexing the converter of the controller is made to improve the current waveform.
In the system indicated in above Patent Publication, a secondary chopper circuit is required, thus a problem arises that the circuit constitution is complicated. Further, the system is structured to add the secondary chopper to the conventional Scherbius apparatus, so that a problem arises that the conversion efficiency is lowered. Furthermore, the multiplexing of the converter causes very complication to the circuit constitution.